You are just answering how the current system actually works. This discussion was about how a hypothetical system works based on GPS or radar alone.
reply> Tunnels are controlled by a single block - only one train can ever be allowed access at once, period.
replyAbsolute nonsense. Do you have any idea how long some tunnels are?
There are actually different definitions for what a block is. It can be both a section where you can decide for trains to enter or where you can change their sequence (Zugfolgeabschnitt vs. Zugmeldeabschnitt, don't know the English terms) Also it can be a physical implementation of any constraint between components of different stations.
replyBut yes there does not need to be a limitation on there only being a single train in a tunnel.